I'm not even sure that's necessary, if all you want to do is create terror...?
For instance, read about David Hahn (aka, the "Nuclear Boy Scout") who recently passed away (of what, they don't say):
https://en.wikipedia.org/wiki/David_Hahn
He managed to contaminate his neighborhood using almost nothing but smoke detector components. How difficult would it be to collect smoke detector parts? You can easily find them used for cheap or free at garage sales and thrift stores. If there were a concerted effort to acquire the parts using only cash, it wouldn't take very long to gather up enough of the material...
...at that point, you just need the delivery device, and we all know that isn't difficult to procure or produce (all the parts can be easily bought at Walmart).
TBH, I'm surprised it hasn't happened yet; as a terrorist weapon, it wouldn't matter that it didn't cause mass death via radiation poisoning (not in the near term at least), but that it has the perception to do so. At the very least, it would make people less likely to visit the "hot zone" left over afterward (in a financially important city area it might lead to economic problems).